Dr. Rodigas talked about several topics related to debris disks around stars. The colors and shapes of these disks can tell us about the compositions of planet-forming materials around the host stars and about the possible presence of otherwise unseen planets.

Without any planets in the system, debris disks are thought to take on very simple shapes, so if an astronomer finds a disk that warped or asymmetric, that can be evidence for a planet gravitationally sculpting the disk.

Dr. Rodigas presented the results of theoretical calculations that showed how the width of a sculpted debris disk can be used to put an upper limit on the mass of a planetary companion — very useful if you’re an astronomer trying to decide where to look for planets.

Prof. Hamilton talked about the origin of Saturn’s moon Titan, an unusual satellite in several ways. Titan has a massive nitrogen and methane atmosphere, full of orange photochemical haze (picture at left).

Prof. Hamilton pointed out that that Saturn’s satellite system is also unique among satellite systems of giant planets: unlike the Jovian and Uranian systems, Titan is the only large moon, and it is very far from the next largest moons in the system.

Instead of forming along with its host planet, as the Jovian and Uranian satellites probably did, Prof. Hamilton suggested that several smaller satellites originally formed around Saturn. Then the moons’ orbits destabilized, and the moons collided, merging to form Titan.

This novel hypothesis solves several outstanding questions about Titan and highlights how much we still don’t understand about our own solar system.

Full sky images of dust in the Milky Way from the Pioneer 10/11 IPP data. From http://www.stsci.edu/~kgordon/pioneer_ipp/Pioneer_10_11_IPP.html.

Dr. Gordon spoke about using observations taken at infrared wavelengths to look for heat radiated by dust grains and then using those measurements to determine how hot the dust is and how much there is. The picture at right shows a map of dust in the Milky Way from one of Dr. Gordon’s papers.

Understanding the dust distributed throughout the Milky Way and other galaxies can tell us a lot about stellar evolution, the galaxies themselves, and about the conditions in those galaxies where the dust lives. And at the most fundamental level, it tells us about the planets and even life itself because the Earth and everything on it formed from this star dust.

She spoke about the Kuiper Belt binary object Sila-Nunam, two enigmatic bodies orbiting 40 times farther from the Sun than the Earth. They have radii of about 100 km, comparable to some of Saturn’s small moons, and they orbit one another every 12 days, as they both go around the Sun together every 300 years.

In recent and coming years, Sila and Nunam will occult one another several times, allowing astronomers to measure their radii, which aren’t very well known, and learn about their densities, internal structures, and orbit.

Dr. Verbiscer presented several very interesting infrared spectra and observations in visible wavelengths, showing the small dips in light from the system, as one object blocked out the other object. These observations are very challenging because objects are so small and far away, but analysis of these data are ongoing and will tell us about these strange, distant, and cold objects.

At a subduction zone, one plate of oceanic lithosphere dives under another plate, which ‘dewaters’ to plate (blue arrows) into the overlying mantle wedge and produces arc volcanism at the surface. Part of the hydrated mantle wedge frees itself and mixes into surrounding depleted mantle. From Widom, Nature 443, 516-517 (2006).

One element that is particularly important for her studies is lithium (Li). Dr. Tian described how Li is thought to behave chemically within the mantle in a way that allows her to trace the subduction of material into the mantle and show that it is later erupted at a mid-ocean ridge (illustrated at right).

To get a sense for how impressive these detections are, consider the following: given the temperature and radius for the host star WASP-17, which is about 1,000 lightyears away, we receive about 1 pico-Watt per square meter from the star here on Earth [ (1.38 R_sun/1000 lightyears)^2 (sigma) (6509 K)^4 ~ 1 pW/m^2].

The planet WASP-17 b has a radius roughly a tenth that of its host star, giving a transit depth [(radius of the planet)^2/(radius of the star)^2] of about 1% (as seen in the figure). For comparison, the radius of a standard lightbulb is about 3 cm and that of a fruitfly is about 2 mm.

So being able to measure a spectrum for WASP-17 b in-transit is a bit like watching a fruitfly pass in front of a lit lightbulb at a distance of 10,000 km from Earth and being able to tell what color the fly’s wings are. Very cool stuff.